Triangular dither-tuned microwave tube

ABSTRACT

A dither-tuned microwave tube apparatus having approximately triangular frequency variation with time includes a movable, frequency controlling tuning element for an electromagnetic cavity. The tuning element is reciprocally moved between two extremes of travel in response to a single rotation of a shaft. A gear train means drives the shaft in a manner such that the tuning element rapidly reverses as it approaches each extreme of travel. The gear train means has two cycles of speed variations during each reciprocation cycle of the tuning element. In one embodiment the gear train means comprises an identical pair of meshing centrally driven elliptical gears, while in a second embodiment the gear train means comprises an identical pair of elliptical gears driven at their foci and a 2:1 gear reduction.

United States Patent 11 1 [111 3,869,638

Gerard Mar. 4, 1975 1 TRIANGULAR DITHER-TUNED Primary Examiner-James W.Lawrence MICROWAVE TUBE Assistant E.\'aminerSaxfield Chatmon, Jr.Attorney, Agent, or FirmStanley Z. Cole; D. R. Pressman; Robert K.Stoddard [57] ABSTRACT A dither-tuned microwave tube apparatus havingapproximately triangular frequency variation with time includes amovable, frequency controlling tuning element for an electromagneticcavity. The tuning element is reciprocally moved between two extremes oftravel in response to a single rotation of a shaft, A gear train meansdrives the shaft in a manner such that the tuning element rapidlyreverses as it approaches each extreme of travel. The gear train meanshas two cycles of speed variations during each reciprocation cycle ofthe tuning element. In one embodiment the gear train means comprises anidentical pair of meshing centrally driven elliptical gears, while in asecond embodiment the gear train means comprises an identical pair ofelliptical gears driven at their foci and a 2:1 gear reduction.

12 Claims, 12 Drawing Figures 3g; la n, I 6

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T T I 0 w n V AEL DU 0 7 m T. 0 I I M GK NN NM w W 2 2 G W R l E A8 V SNIL NA 0 NSECL s W CL 0 \|\R C 6 Mu l T2 m A. m l M 0 FM M DI TUNER LCRANK MEANS COMPENSATING 3 TRIANCUL- ARIZING MEANS S 24 MOTOR TRIANGULARDITHER-TUNED MICROWAVE TUBE FIELD OF THE INVENTION BACKGROUND OF THEINVENTION Heretofore, dither-tuned microwave tubes, particularlymagnetrons, have frequently employed a substantially constant speedmotor driven crankshaft reciprocating a tuning plunger between twoextremes within a cavity of the tube. The plunger reciprocatessubstantially sinusoidally in time, typically at 100 to 200 hertz,thereby producing a sinusoidal variation in the output carrier frequencyof the tube. Such a dither-tuned microwave tube is used for providing aband of carrier frequencies for transmitted pulses in a frequency agileradar. Because of the sinusoidal carrier frequency variation, there isan undesirable increased .carrier frequency density at the extremecarrier frequencies which are derived at the travel extremes of thetuning plunger. Since the pulse repetition rate of such a radar isconstant, more pulses have carrier frequencies near the extremes, wherethe sinusoidally varying frequency is changing slower in time, than inthe frequency band centered between the extremes. Thus, adjacent pulsesat the extreme carrier frequencies do not differ from each othersufficiently to be considered as having independent carrier frequencies,with a tendency to preclude the expected advantages of frequency agileradar. Further, countermeasures against such a radar can be moreeffective because the frequency spectrum of the radar is concentrated atthe frequency extremes. These undesirable characteristics inhere in thesinusoidal frequency variation of the present generally availabledither-tuned microwave tubes of the type employing a crankshaft.

While the crankshaft type system has these disadvantages, it has beenfound to be particularly adaptable to precise plunger position control.Additionally, the instantaneous angle of the rotatable crankshaft iseasily and directly read out with an electromechanical resolver thatexhibits a sinusoidal voltage envelope variation with crankshaft angle.The resolver provides an output voltage proportional to instantaneousfrequency which is also a substantially sinusoidal function ofcrankshaft angle.

A triangular variation of the operating frequency in time (Le, a linearrise and fall) does not present these objectionable characteristicsbecause the undesirable increase of pulses at the carrier frequencyextremes is eliminated. However, a triangular frequency variation canonly be approached because it implies infinite deceleration andacceleration of the tuning plunger at the extremes of travel to reversethe plunger direction, i.e., to instantaneously change the plunger froma positive velocity to a negative velocity.

OBJECTS OF THE PRESENT INVENTION It is an object of the presentinvention to provide a new and improved dither-tuned microwave tubeexhibiting a substantially triangular variation of microwave outputfrequency with time.

It is another object of the present invention to provide a dither-tunedmicrowave tube having a crankshaft for reciprocating a tuning plungerbetween two extremes, thereby varying the operating frequency of thetube between two extremes, with means for driving the crankshaft toproduce a substantially triangular variation of the output frequencywith time.

It is another object of the present invention to provide a dither-tunedmicrowave tube having a crankshaft for reciprocating a tuning plungerbetween two extremes, thereby varying the operating frequency of thetube between the two extremes, with means for rapidly decelerating thetuning plunger at the extremes.

SUMMARY OF THE PRESENT INVENTION These objects are fulfilled byproviding a non-linear gear train. to drive a crankshaft for a tuningplunger of a dither-tuned microwave tube. The gear train causes a morerapid rotation of the crankshaft at the extremes of tuning plungertravel than midway between the extremes. The gear train thus functionsas means for producing two cycles of crankshaft speed variation for eachcycle of the tuning plunger reciprocation to provide maximum speed atthe two extremes, and minimum speed at each of two crossings through apoint midway between the extremes. The gear train, in one embodiment,comprises a pair of meshing identical center rotated elliptical gears.which produce two cycles of speed variation per complete gear rotation.The pair of elliptical gears has equivalents, such as other non-linearor eccentric gear arrangements for producing a different number of speedvariations per gear rotation combined with an appropriate gear ratio forproducing two speed variations for each complete cycle of crankshaftreciprocation.

Other objects, features and advantages of the present invention willbecome apparent from the following de tailed description of theinvention, taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a mechanical schematicdiagram of a prior art dither-tuned microwave tube system having a crankdriven tuning plunger;

FIG. 2 is a mechanical schematic diagram of the dither-tuned microwavetube of the invention having triangularizing means for driving thetuning plunger to ob tain a substantially triangular variation of tubeoperating frequency;

FIG. 3 is a cross-sectional view of a preferred embodiment of thedithentuned microwave tube of the invention schematically illustrated inFIG. 2;

FIGS. 4, 5 and 6 are views of FIG. 3 along the lines 44, 5-5, and 6-6,respectively, with: FIG. 4 showing a pair of centrally driven ellipticalgears forming the triangularizing. means, FIG. 5 showing a pair of focusdriven eccentric gears, and FIG. 6 showing an eccentric bearing forcranking the tuning plunger;

FIG. 7 is a view of the centrally driven elliptical gears at a positiondisplaced from the position shown in FIG. 4;

FIG. 8 is a schematic diagram showing an alternate embodiment forderiving a substantially triangular variation of tube operatingfrequency; and

sus time, tuning plunger crankshaft angle of rotation versus time, andthe tube operating frequency versus time.

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1, there isshown schematically a prior art drive for a tuning plunger 19 of adither-tuned microwave tube which is preferably a magnetron. The tuningplunger 19 is ultimately driven by constant speed motor 12 whichdirectly drives input shaft 14. Compensating means is driven by inputshaft 14 and drives tuning crank 18 through rotating crankshaft 22.Tuning crank 18 reciprocally slides tuning plunger 19 within microwavetube 10. Shaft 14 also drives a resolver 16, for providing electricalreadout of the tube operating frequency. The constant rotational speedof shaft 14 produces a sinusoidal reciprocating movement of tuningplunger 19 that is approximately tracked by a sinusoidal output voltageof resolver 16.

Compensating means 20 drives tuner crankshaft 22 and plunger 19 tocompensate for non-linearity of the relationship between theinstantaneous position of the plunger and the microwave carrierfrequency derived from tube 10. Compensation is necessary to produce asubstantially pure sinusoidal operating frequency variation from thetube 10 for constant rotation rate of shaft 14 in order for the resolveroutput voltage to accurately track the operating frequency of the tube.The compensating means 18, invented by Richard C. Stoke, can be furtherstudied in US. Pat. No. 3,590,313, issued June 29, 1971, although thefollowing brief discussion of it suffices for the purposes of thepresent invention.

Non-linearity in the relationship between the position, of tuningplunger 19 and the operating frequency of tube 10 has a tendency tocause the time for the operating frequency to vary between one extremeand a median value between the extremes to be less than the timerequired to change from the median value to the opposite extreme. Thus,driving tuner crankshaft 22 at constant speed has a tendency to producea microwave frequency asymmetry between alternate half cycles of therotation of shaft 14. Compensating means 20 overcomes this tendency bydriving the tuner crankshaft 22 slower during the half cycle of shaft 14that is relatively short, while during the longer half cycle of shaft 14the compensating means drives shaft 22 faster than driving thepreceeding half cycle. Thus, the compensating means 20 drives shaft 22so that the shaft has only one cycle of speed variation per rotation oftuner crankshaft 19.

Reference is now made to FIG. 2, a schematic diagram of a preferredembodiment of the invention wherein the device of FIG. 1 is modified byincluding triangularizing means 24,, driven by constant speed motor 12via shaft 26, for driving input shaft 14 at a cyclically varying speed.Triangularizing means 24 rotates shaft 14 more rapidly at the extremesof travel of tuning plunger 19 than midway between the extremes in orderto produce a nearly triangular variation with time, i.e., linearoppositely directed variations during adjacent half cycles, of both theoperating frequency of tube 10 and the frequency tracking output voltageV of resolver 16. The triangularizing means 24 must therefore producetwo cycles of speed variation for each complete rotation of tunercrankshaft 22 and each corresponding complete reciprocation of tuningplunger Reference is now made to FIG. 3, wherein there is shown-in crosssection the operative parts of one embodiment of the dither-tunedmicrowave tube 10 of the invention. Tube 10 is a magnetron tube of thetype generally described in US. Pat. No. 3,441 ,795, issued Apr. 29,I969. Tube 10 includes an annular resonant cavity 30, defined by anouter cylindrical wall 31, an inner cylindrical wall 36 and upper andlower end walls 40 and 42. An array of vane resonators 32 is directedinwardly from inner cylindrical wall 36 and coaxially surrounds an axialcathode 34 to define an annular magnetron interaction region between thevane resonators and the cathode. Wall 36 is a boundary between the innerannular magnetron interaction region and the outer annular cavityresonator. An array of coupling slots 38 in wall 36 between alternatevane resonators 32 is provided for coupling the 1r mode of oscillationin the magnetron region to a-circular electric oscillating mode in theannular cavity resonator. The upper end wall 40 of the annular cavity ismovable between upper limit 41 and lower limit 43 to effect tuning ofthe annular cavity 30 and to thus vary the operating frequency of thetube. Movable wall 40 is the lower surface of tuning plunger 19 which isvertically reciprocated by tuning plunger crank 18, thereby producing acyclical operating frequency variation with time. i

The tuning crank 18 comprises a slider crank mechanism including apiston rod 44 having a lower end coupled to tuning plunger 19 via balljoint 46. The upper end of rod 44, as indicated in FIG. 6, forms acircular yoke 48 housing a ball bearing race 50. Within ball bearingrace 50 is a circular eccentric portion'52 at one end of tunercrankshaft 22. It is a well known characteristic of a slider-crankmechanism that a constant rotation of the crankshaft produces asubstantially sinusoidal reciprocation of the slider, in this case thetuning plunger 19.

Tuner crankshaft 22 is cylindrical and hollow enabling resolver inputshaft 54 to pass coaxially through the crankshaft for driving resolver16. The coaxial arrangement of shafts 22 and 54 enable resolver 16 to belocated for tight packaging of the entire dither-tuned microwave tubeassembly. The rotor of resolver 16 is connected to shaft 54 which isdriven from input shaft 14, via gears 56 and 58 having a 1:1 ratio. Itshould be understood that the resolver position could be such that therotor is directly attached to shaft 14.

.Compensating means 20, illustrated in FIG. 5, comprises a pair ofmeshing gears 60 and 62 having axes or foci 61 and 63, respectively.Shafts 22 and 14 (FIG. 3), are fixedly connected in driving relationwith gears 60 and 62 for rotation of the gears about their respectiveaxes. Since meshing gears 60 and 62 are identical, one

complete rotation of input gear 60 and shaft 14 provides a completerotation of the output gear 62 and shaft 22, corresponding to an averagegear ratio 1:1. Output gear 62 drives crankshaft 22 and hence eccentricportion 52 to cause tuning plunger 19 to move from one limit of itstravel to the other limit of its travel as input shaft 14 rotates. Dueto movement of plunger 19, there is a sinusoidal variation of operatingfrequency which is accurately read out by resolver 16.

Since it is desired to provide, as closely as possible, a triangularvariation of operating frequency versus time, shaft 14 is not directlydriven by a constantly rotating shaft 26; instead, triangularizing means24 is interposed between shafts 26 and 14. Triangularizing means 24drives the input shaft 14 more rapidly when the tuning plunger 19 is atthe limits 41 and 43 of its travel than when the plunger is midwaybetween these limits. The triangularizing means comprises an identicalpair of meshing elliptical gears 70 and 72 (FIG. 4) which are fixedlyand coaxially mounted to shafts 26 and 14 respectively for rotationabout the respective gear centers 74 and 76. Shafts 26 and 14 are spacedapart a distance equal to the sum of the semi-major and semi-minor axesof the ellipses forming the gears. The distances from the point 82 wherethe gears mesh to the centers of gears 70 and 72 are respectivelyrepresented by lines 78 and 80. Since the ratio of the distancesrepresented by lines 78 and 80 changes for each angular position ofshaft 26 and gear 72, there is a constant speed variation in output gear72 when input gear 70 is constantly rotated. The average speed of outputgear 72 equals the average speed of input gear since the gears areidentical, whereby a complete rotation of the input gear produces acomplete rotation of the output sitions (one shown in FIG. 4 and anothershown in FIG.

7) where the major axis of one gear is aligned with the minor axis ofthe othergear. When the distance represented by line 78 is greater thanthe distance represented by 80, as shown in FIG. 4, there is anincreased speed of output gear 72 over average. However, after 90 ofrotation of gears 70 and 72, as shown in FIG. 7, the distancesrepresented by lines 80 and 82 are reversed and the output gear 72 has alower speed than average. Thus, there are two maxima and two minima inthe rotation rate of gear 72 and shaft 14 for one revolution of gears 70and 72 as well as shafts 26 and 14. The maximum rotation rate of gear 72is determined by multiplying the constant speed rotation rate of shaft26 by the ratio of the major axis to the minor axis of the identicalelliptical gears 70 and 72. In contrast, the minimum rotation rate ofgear 72 is found by dividing the speed of shaft 26 by the same ratio.

It should be understood that there are numerous equivalents fortriangularizing means 24. For example, as shown in FIG. 8, one cyclemeans 84, such as a pair of focus rotated elliptical gears, can be usedto drive a 2:1 reduction gear which in turn drives the input shaft 14whereby two revolutions of the focus rotated elliptical gears andconsequently two speed cycles are produced for each revolution of inputshaft 14.

.Referring now to FIG. 9a, there is a plot 90 relating operating carrierfrequency, f, and the instantaneous angle 6 of input shaft 14. Thefrequency variation is a substantially pure sinusoidal function of theangular position of shaft 14, due in part to the provision of thecompensating means 20 interposed-between shaft 14 and the tuner crank18. As is apparent, a constant rotation rate of shaft 14 producesfrequency the same sinusoidal function of time. Plot 92, in FIG. 9b,relates the instanteous speed, d0/a't, of input shaft 14 versus time asproduced by the triangularizing means 24 where T is the pcriod for onerevolution of the input shaft. Plot 92 is a periodic waveform with twocycles in the period T in which the undirectional dB/dt varies about anaverage value of d6/dt represented by horizontal line 110. d6/dt has aminimum value 112 occuring at zero time. Another plot, 94, in FIG. 9cshows the angle6 as ordinate versus time, where 0 is obtained byintegrating plot 92 and initializing 0 equal to 0 and time O. 0 equal to0 corresponds to a tuning plunger 19 location midway between theextremes of travel 41 and 43. Plot 94 shows a two cycle periodicundulation of 6 about a straight line 114. This straight line 114corresponds to a linear ascent of 0 in accordance with the average value110 of d0/dt. Because of plot 92s two cycle periodicity, 6 correspondsto 180 at time equal to half of the period and 360 at time equal to thecomplete period. With this information, the resultant fre quency versustime plot 96 can be constructed. As can be seen, in the region of thefrequency versus 0 near 0, 0 changes slowly, because of proximity to theminimum value 112, while in the region of the frequency versus 0 curvenear 0 changes rapidly thereby producing the substantially triangularvariation 96 which is characterized by generally straight rising andfalling portions 98 between the short curved portions 100 at the maximumand minimum frequencies. The curved portions 100 have a radius ofcurvature substantially less than that of a sine wave of the samefrequency excursion and period T. This radius of curvature is related tothe turn-around deceleration of the tuning plunger; the smaller theradius of curvature, the higher the turnaround deceleration.

It has been found that the tuning plunger of a sinusoidally dither-tunedX-band magnetron normally exhibiting 3G peak turn-around decelerationcan be driven with the present invention at decelerations up to 46 G5with no bearing failure. This corresponds approximately to a decrease inthe radius of curvature at the extremes of the frequency versus timecurve by a factor of 15.

As should be apparent, having described the invention, numerousmodifications are possible within its spirit and scope. For example thegear train means can also be used to drive the ceramic element in amagnetron of the type which is dithered by rotation of such an elementin its cavity or to drive the band used to constrict the flexible wallof the cavity of a magnetron which is dithered by tightening andloosening such a belly band around its cavity. Also the gear train meanscan be used to drive the tuning elements of other microwave tubes. It istherefore intended that the spirit and scope of the invention bedetermined with reference to the appended claims.

What is claimed is:

1. In combination with a tunable frequency microwave tube apparatushaving microwave circuit means, tuning means movable within the regionof said circuit means for tuning said microwave circuit means, and thusthe operating frequency of the tube; an input shaft; means forcyclically moving said tuning means between two extremes of travel inresponse to rotation of said input shaft dither the operating frequency;the improvement comprising first gear train means driving said inputshaft from a drive shaft and producing with respect to rotation of saiddrive shaft a cyclical rotation rate of said input shaft such thatmaximum rotation rate occurs at said two extremes of travel of saidtuning means and such that the operating frequency of said tube has atriangular dither characteristic with respect to time.

2. The apparatus of claim I wherein said cyclically moving meansincludes second gear train means cou pling said first gear train meansto said inputshaft and producing, with respect to rotation of the outputshaft of said first gear train means, rotation of said input shafthaving one cyclic variation of rate per one cycle of motion of saidtuning means.

3. The apparatus of claim 1 wherein one cycle of rotation of said inputcauses one cycle of motion of said tuning means and said first geartrain means includes means for causing a two cycle speed variation inthe movement of said input shaft for each cycle of movement of saidtuning means.

4. The apparatus of claim 2 wherein one cycle of rotation of said inputshaft produces one cycle of movement of said tuning means.

5. The apparatus of claim 1 wherein said first gear train meanscomprises an identical pair of meshing elliptical gears.

6. The apparatus of claim 5 wherein said pair of meshing ellipticalgears has centers and wherein said first gear train means additionallycomprises means for rotating the elliptical gears about their centers.

7. The apparatus of claim 2 including additional readout means coupledto said output shaft of said first gear train for indicating theinstantaneous angle of said shaft and thus said operating frequency.

8. A microwave tube structure comprising a cavity, having a resonantfrequency, tuning means within said cavity for varying the resonantfrequency of said cavity, a shaft, means for cyclically moving saidtuning means between two extremes of travel in response to rotation ofsaid shaft, and means for driving said shaft with a cyclical rate ofrotation, which rate is maximum at the two extremes of travel of thetuning means and such that the operating frequency of said tube has atriangular dither characteristic with respect to time.

9. The structure of claim 8 wherein said driving means comprises anidentical pair of meshing elliptical gears.

10. The structure of claim 9 wherein said pair of meshing ellipticalgears has centers and wherein said driving means additionally comprisesmeans for rotating the elliptical gears about their centers.

11. The structure of claim 9 additionally comprising a magnetroninteraction region coupled to said cavity.

12. The apparatus of claim 2 wherein said second gear train meanscomprises a pair of eccentric gears.

1. In combination with a tunable frequency microwave tube apparatushaving microwave circuit means, tuning means movable within the regionof said circuit means for tuning said microwave circuit means, and thusthe operating frequency of the tube; an input shaft; means forcyclically moving said tuning means between two extremes of travel inresponse to rotation of said input shaft dither the operating frequency;the improvement comprising first gear train means driving said inputshaft from a drive shaft and producing with respect to rotation of saiddrive shaft a cyclical rotation rate of said input shaft such thatmaximum rotation rate occurs at said two extremes of travel of saidtuning means and such that the operating frequency of said tube has atriangular dither characteristic with respect to time.
 2. The apparatusof claim 1 wherein said cyclically moving means includes second geartrain means coupling said first gear train means to said input shaft andproducing, with respect to rotation of the output shaft of said firstgear train means, rotation of said input shaft having one cyclicvariation of rate per one cycle of motion of said tuning means.
 3. Theapparatus of claim 1 wherein one cycle of rotation of said input causesone cycle of motion of said tuning means and said first gear train meansincludes means for causing a two cycle speed variation in the movementof said input shaft for each cycle of movement of said tuning means. 4.The apparatus of claim 2 wherein one cycle of rotation of said inputshaft produces one cycle of movement of said tuning means.
 5. Theapparatus of claim 1 wherein said first gear train means comprises anidentical pair of meshing elliptical gears.
 6. The apparatus of claim 5wherein said pair of meshing elliptical gears has centers and whereinsaid first gear train means additionally comprises means for rotatingthe elliptical gears about their centers.
 7. The apparatus of claim 2including additional readout means coupled to said output shaft of saidfirst gear train for indicating the instantaneous angle of said shaftand thus said operating frequency.
 8. A microwave tube structurecomprising a cavity, having a resonant frequency, tuning means withinsaid cavity for varying the resonant frequency of said cavity, a shaft,means for cyclically moving said tuning means between two extremes oftravel in response to rotation of said shaft, and means for driving saidshaft with a cyclical rate of rotation, which rate is maximum at the twoextremes of travel of the tuning means and such that the operatingfrequency of said tube has a triangular dither characterIstic withrespect to time.
 9. The structure of claim 8 wherein said driving meanscomprises an identical pair of meshing elliptical gears.
 10. Thestructure of claim 9 wherein said pair of meshing elliptical gears hascenters and wherein said driving means additionally comprises means forrotating the elliptical gears about their centers.
 11. The structure ofclaim 9 additionally comprising a magnetron interaction region coupledto said cavity.
 12. The apparatus of claim 2 wherein said second geartrain means comprises a pair of eccentric gears.